Archive of Science Results - Ulysses
Ulysses Science Results
- Solar Wind at a 50-Year Low, September 2008
- Comet McNaught, May 2007
- Suprathermal Ion Tails, February 2007
- Ulysses: Back in the Fast Solar Wind From the Southern Polar Coronal Hole, May 2006
- Localized "Jets" of Jovian Electrons Observed During Ulysses' Distant Jupiter Flyby in 2003-2004, February 2006
- Solar Activity Associated With Active Region 10808 in August and September 2005 as Seen From Ulysses, November 2005
- Ulysses Observations of the Hard X-ray Emission From the Giant Solar Flare on 4 November 2003, May 2005
- Identifying the Same Very Hot, Highly Ionised Plasma (6-10 MK) Remotely at the Sun With SOHO and, After Propagation, in Situ at Ulysses, February 2005
- 27 September, 2004: "Shields Up!"
- 14 September, 2004: "Beware: Io Dust"
- 17 March, 2004: "Cold Peril: The Continuing Adventures of Ulysses"
- 22 April, 2003: "A Star with two North Poles"
- Ulysses Observations of Cross-Field Diffusion Flow of Solar Energetic Particles at High Latitudes, April 2003
- 8 January, 2003: "Solar Spitwads"
- Modeling the Heliospheric Current Sheet: Solar Cycle Variations, July 2002
- Solar Energetic Particles - Wide Angular Distribution About the Sun, March 2001
- Composition and Charge State of the Solar Wind, February 2001
- Rigidity Dependence of Cosmic Ray Proton Latitudinal Gradients, January 2001
- Asymmetric Solar Magnetic Field, November 2000
- Solar Energetic Particles: Multiple Injections of Particles From CMEs Produce Elevated Minimum Fluxes at 5 AU During 1998, October 2000
- Origin of Inner Source Pickup Ions Discovered, September 2000
- Ulysses Observes Irregular Solar Wind as Solar Maximum Approaches, August 2000
- Interplanetary Network for Gamma-Ray Burst Location, June 2000
- Ulysses Encounter with Comet Hyakutake, April 2000
- Speed limit for Proton Beams in the Solar Wind, March 2000
- New Understanding of Solar Radio Bursts, January 2000
- Charge Exchange between Interstellar Neutrals and the Solar Wind, November 1999
- Cometary Tail Disconnection Events, October 1999
- Ulysses Radio Observations of Jupiter's Space Weather, July 1999
Further details available here
- Ulysses Measures Consistent Ages for Cosmic Rays Using Four Radioactive Clocks, June 1999
Further details available here
- Upturn in Solar Activity - Ulysses Magnetic Field April 1999
- Energetic Particle Propagation, March 1999
- Population of Large Dust Particles Discovered by Ulysses, January 1999
- Ulysses Captures Gamma-Ray Flare from Magnetic Star, December 1998
Further details available here
- Energetic Ion Abundances at Corotating Reverse Shocks, October 1998
- Long Mean Free Path for Pick-up Ions in the High-Latitude Solar Wind, August 1998
- Radial Dependence of Electron Density and Temperature in the High-Latitude Solar Wind, June 1998
- Determination of Gamma-Ray Burst Location by Ulysses, May 1998
- Boundary Between Fast and Slow Solar Wind, April 1998
- Cosmic Rays in the Galactic Halo, March 1998
- Shock Distributions, January 1998
Ulysses at Solar Maximum and Beyond
The mission is now in its thirteenth year, and all spacecraft systems and the nine sets of scientific instruments remain in excellent health. Ulysses arrived over the sun's south polar regions for the second time in November 2000, followed by the rapid transit from maximum southern to maximum northern helio-latitudes that was completed in October 2001. Solar activity reached its maximum in 2000, so that Ulysses experienced a very different high-latitude environment from the one it encountered during the first high-latitude passes. The spacecraft is now heading away from the sun towards aphelion at the end of June 2004.
At its meeting in June 2000, ESA's Science Programme Committee approved the continuation of mission until September 2004, when Ulysses will have completed two full out-of-ecliptic orbits of the sun. Based on the positive recommendation of the Sun Earth Connections Senior Review held in mid-2001, NASA has also confirmed its support for mission operations until 2004.
With the completion of the second northern polar pass in December 2001, Ulysses has provided the first, and for the forseeable future only, survey of the high-latitude heliosphere within 5 AU of the sun over the full range of solar activity conditions. The results from the first set of polar passes, in 1994-95, have been described extensively in the scientific literature, in earlier COSPAR reports, and in particular in the recently published book "The Heliosphere Near Solar Minimum: the Ulysses Perspective". Preliminary results from the polar passes at solar maximum are summarised in the proceedings of the 34th ESLAB Symposium, "The 3-D Heliosphere at Solar Maximum". Here, we focus on some of the recent highlights.
In order to provide context for some of the other results, we first discuss the solar wind and magnetic field observations. The exploration of the latitude dependence of solar wind characteristics (speed, temperature, composition) during maximum solar activity has revealed an entirely different configuration of the 3-dimensional heliosphere compared with that observed near solar minimum. At solar minimum, Ulysses found a heliosphere dominated by the fast wind from the southern and northern polar coronal holes. In contrast, during solar maximum, the large polar coronal holes had disappeared, and the heliosphere appeared much more symmetric. The solar wind flows measured throughout the south polar pass, and much of the rapid transit from south to north, showed no systematic dependence on latitude. The wind itself was generally slower and much more variable than at solar minimum at all latitudes. Nevertheless, when Ulysses reached high northern latitudes in late 2001, it witnessed the formation and growth of a new polar coronal hole. This clearly marked the return to more stable conditions following activity maximum. The solar wind recorded at Ulysses became commensurately faster and more uniform, resembling the flows seen over the poles at solar minimum.
The development of the solar wind characteristics was matched by the magnetic field observations. One of the questions that has been investigated is how the high solar activity level affected the structure and dynamics of the heliospheric magnetic field. Although the solar magnetic field, corona and solar wind were highly variable, the magnetic field at Ulysses (~1.5-2.5 AU from the sun) maintained a surprisingly simple, dipole-like structure. In contrast to the situation at solar minimum, however, the equivalent magnetic poles were located at low latitudes rather than in the polar caps. This is consistent with the presence of coronal holes (the source of the open field carried out with the solar wind to form the heliospheric magnetic field) near the equator, and their absence at the poles. In turn, the spreading out of the field lines from these equatorial sources to high latitudes caused the solar wind to be deflected pole-ward.
Another phenomenon of great interest was the sun's magnetic polarity reversal that occurred during the polar passes. It was found that the process of magnetic polarity reversal is a complex one that takes place over a period of several months while the corona evolves to reflect the changes occurring at the level of the photosphere. The coronal evolution is reflected, in turn, in complex dynamic structures in the heliospheric magnetic field. Nevertheless, the previous finding of Ulysses concerning the heliolatitude independence of the radial component of the magnetic field at solar minimum is found to be valid even in the more disturbed conditions at solar maximum. This therefore appears to be a fundamental property of the distribution of magnetic flux carried in the solar wind.
One of the key discoveries made during the high-latitude passes at solar minimum was the unexpected ease with which energetic particles, accelerated at low-to-middle latitudes, were able to gain access to the polar regions of the heliosphere. Recurrent increases in particle intensity, with a period of ~26 days (i.e., close to the solar rotation period), were observed up to the highest latitudes even though the source of these particles, so-called corotating interaction regions (CIRs), were confined to much lower latitudes. This discovery prompted theorists to re-assess existing models of the heliospheric magnetic field, leading ultimately to new suggestions regarding the source of the solar wind itself. An obvious question, then, when Ulysses returned to high latitudes at solar maximum, was: do energetic particles have the same easy access when the heliosphere is much more chaotic? The observations provided an unequivocal answer: yes. Clearly, the source of energetic particles at solar maximum is not CIR-related, since the stable fast-slow solar wind stream structures that are responsible for CIRs do not exist during this phase of the solar cycle. Transient shock waves that are driven by fast coronal mass ejections (CMEs) give rise to the numerous large increases in particle flux that are characteristic of solar maximum. The Ulysses data from the recent high-latitude passes, in addition to confirming the presence of large fluxes of energetic particles over the poles, have revealed that the absolute intensity of these particles in many events is comparable to that measured simultaneously in the ecliptic near 1 AU. This has lead to the idea of the inner heliosphere near solar maximum acting as a "reservoir" for solar energetic particles.
The precise mechanism by which the particles are transported in latitude and longitude to fill this reservoir is still being debated. The process is certainly different from that in operation at solar minimum, whereby systematic changes to the underlying Parker spiral magnetic field configuration permit particles accelerated at low-latitude CIRs to propagate to an observer at high latitudes. As noted above, at solar maximum, magnetic pressure acting on open magnetic field lines originating in near-equatorial coronal holes may push them pole-ward, thereby providing the necessary magnetic connection to high latitudes. Whatever the mechanism, Ulysses has shown that energetic charged particles can gain access to the high-latitude regions of the heliosphere at all phases of the solar cycle far more easily than expected.
On a different topic, Ulysses has for the first time directly observed how the magnetic field of the sun changes the amount of extra-solar material entering the solar system. The dust detector has counted an increasing number of interstellar grains since the beginning of 2000. This increase follows a depletion of interstellar grains observed by Ulysses after mid-1996 that was attributed to the deflection of the interstellar dust stream by the highly ordered, solar-minimum heliospheric magnetic field. According to model calculations of the electromagnetic interaction of small interstellar dust grains with the heliospheric plasma, the return of interstellar dust to the solar system is the result of large-scale disturbances of the heliospheric magnetic field that occur during solar maximum. The model calculations also show that the configuration of the heliospheric magnetic field will cause the number of interstellar dust grains in the solar system to steadily increase until the next solar maximum.
Ulysses Returns to the Sun's South Pole
On 8 September, the European-built space probe Ulysses will begin its second passage over the Sun's south pole. Launched in October 1990, Ulysses first flew over the Sun's south pole in 1994, followed one year later by a visit to the north pole. In the 10 years since launch, the instruments on board Ulysses have provided scientists with a unique picture of the Sun's environment, the heliosphere. Although Ulysses is travelling along the same path it followed 6 years ago, there is no sense of 'déjà-vu' when examining recent data. The reason is that one of the key players in Ulysses' adventure, the Sun, has undergone a dramatic transformation in the meantime. Solar activity, related to the magnetic behaviour of the star, was very low in 1994. The solar wind at Ulysses was fast but steady. This time around, the 11-year activity cycle is at its peak. Solar storms are numerous, and the high-latitude solar wind is blustery.
Ulysses will spend four months above 70º southern latitude, before swinging equatorward to turn its attention to the northern hemisphere. During the space probe's sojourn at high latitudes, scientists will be eager to learn just how different the polar regions of the Sun are at solar maximum compared with activity minimum. Aside from the solar wind, they will be measuring the influx of cosmic-ray particles and the complex electric and magnetic fields that constrain their trajectories, as well as searching for interstellar dust and gas. Ulysses will also continue its role as a monitoring station for ubiquitous gamma-ray bursts.
One of the most successful interplanetary missions ever launched, Ulysses has already changed our view of the heliosphere for ever. Among the key findings at solar minimum was the discovery that the fast solar wind from the solar poles fills a large fraction of the heliosphere, blowing steadily at around 750 km/s. Ulysses' state-of-the-art instruments were also able to show that the boundary between the fast wind and the slower, more variable wind from the equatorial regions is surprisingly sharp. Another surprise was that the effects of collisions between fast and slow wind streams occurring at low latitudes continue to be felt up to the poles. The scientific explorations carried out by Ulysses have not been confined to the Sun and its heliosphere. Instruments on board the spacecraft have made the first-ever measurements of dust particles and neutral helium atoms of interstellar origin. These findings, together with the detection of other constituents of interstellar gas that have become ionised in the solar wind, have lead to a major increase in our knowledge of the gas and dust clouds surrounding the heliosphere. High-precision measurements of the isotopes of cosmic ray nuclei have lead to a better understanding of processes occurring in distant supernova- explosions.
If there is one lesson that Ulysses has taught us during the past ten years, it is that while some of our expectations regarding the global nature of the heliosphere may be borne out by the observations, there is always room for surprises. If this holds for the relative simplicity of solar minimum, it will surely be true at solar maximum, where even our preconceptions are less well formed. One thing is sure: there are exciting times ahead in our quest to understand the Sun and its heliosphere.
ESA's Science Programme Committee approves Ulysses extension
An important step has been taken towards granting the joint ESA-NASA Ulysses mission an extra 2 years 9 months of life. At its meeting in Paris on 5-6 June, ESA's Science Programme Committee approved the continuation of orbital operations from the end of 2001 to 30 September 2004. If NASA follows ESA's lead, the extension will allow Ulysses to observe the Sun's environment as sunspot activity gradually declines after this year's sunspot maximum.
Over the past ten years, Ulysses has made many remarkable discoveries about the heliosphere, the vast bubble blown out into space by the solar wind, from its unique solar polar orbit. The extension to 30 September 2004 would allow the spacecraft to complete two full orbits around the poles of the Sun and make further discoveries throughout a full 11-year solar cycle. "Ulysses has already changed our view of the heliosphere in many fundamental ways. We are delighted that the SPC has approved the extension and we are now looking forward to a positive decision from NASA," says Richard Marsden, ESA's Ulysses Project Scientist.
Around solar maximum, the polarity of the Sun's magnetic field reverses, causing corresponding changes to the heliosphere. Ulysses is expected to make new discoveries about how this readjustment takes place in the months following the field reversal. Extending the mission will allow the spacecraft to gain a clear picture of the behaviour of the heliosphere as it settles down once again to quiet times from the Sun. "We'll be watching carefully as the relative chaos of the active Sun makes way for a more stable pattern," says Richard Marsden. " The effects will be most readily seen in the behaviour of cosmic ray particles arriving from outside the heliosphere." Although Ulysses was in orbit during the declining phase of the previous solar cycle, it was en-route to Jupiter, and all measurements were made near the ecliptic (the plane in which the planets orbit) rather than at high solar latitude. The extension would allow Ulysses to make the first-ever set of high-latitude observations over a full solar cycle.
The extension would also allow Ulysses to continue addressing a broad range of astrophysical phenomena, which include locating gamma-ray burst sources, studying the interstellar abundance of rare species like deuterium and 3He, and increasing the precision of cosmic ray isotopic abundance measurements. After a detailed study, ESA concluded that there are sufficient on-board consumables to keep a core payload operating continuously provided there is some time- sharing among other scientific instruments. The additional cost to ESA represents excellent value-for-money in return for a significant enhancement to the scientific harvest of the Ulysses mission. On the NASA side, the mission is already funded until December 2002, and a decision regarding a further extension is expected in mid-2001.
Ulysses feels the brush of a comet's tail
Digital painting courtesy of David A. Hardy
Ulysses has added comet spotter to its list of talents. Two papers published in Nature today report that on 1 May 1996, the spacecraft flew through the tail of comet Hyakutake whose nucleus was more than 3.5AU (one AU equals the Sun-Earth distance) away at the time. "This makes it the longest comet tail ever recorded", says Geraint Jones from Imperial College, London who is a member of one of the two instrument teams that made the discovery. "Ulysses's prime task is to map the solar wind above the Sun's poles: it had not been looking for Hyakutake, which happened to be at its closest approach to the Sun on 1 May 1996, or any other comet," says Richard Marsden, ESA's Ulysses Project Scientist. George Gloeckler from the University of Maryland who is a member of the other instrument team, says: "The discovery was made quite by accident. It was a bit like finding a needle in a haystack when you weren't even looking for a needle in the first place". Jones, Gloeckler and their colleagues stumbled across the telltale signature of a comet quite independently when poring over old Ulysses data.
"I was looking for changes in the ionisation levels of the solar wind that would tell me about unusual solar activity," says Gloeckler who is Principal Investigator of the SWICS (Solar Wind Ion Composition Spectrometer) instrument. "The solar wind normally consists of multiply charged ions. The signature stood out because the number of singly charged ions jumped to several thousand times the background level. I thought this might be due to strange solar eruptions. But when I looked at the composition of the ions, I knew immediately that they were cometary in origin." Cometary tails are rich in oxygen and carbon ions compared with the solar wind, but depleted in nitrogen and neon. Jones and colleagues found their evidence in data from the Ulysses magnetometer.
"The magnetic field lines were draped in a way that you'd expect in a comet's tail. The solar wind is slowed down at the centre of the tail compared with the edge, which gives the magnetic field associated with the travelling ions a characteristic hairpin shape," says Jones. The findings from both teams corroborate an earlier Ulysses discovery, reported in 1998 by Pete Riley and colleagues at the Los Alamos National Laboratory, of a drop in the proton density in the solar wind on 1 May 1996. "They wondered whether the drop could have been due to a comet, but went no further," says Jones. With the evidence now mounting, the Imperial College team decided to look for a comet that would have been in the right place at the right time to account for the Ulysses data. "Hyakutake was the first comet we looked at. When I compared the orbit of Ulysses with the orbit of the comet, I found that Ulysses was extremely close to Hyakutake's orbital plane at the time," says Jones. On 1 May 1996, Ulysses was aligned with the Sun and the position Hyakutake had occupied eight days earlier. Jones calculated that eight days was the time needed for material leaving the comet's nucleus to travel the 3.5AU distance to Ulysses. One of the most surprising aspects of the discovery is the length of Hyakutake's tail, which must have been at least 3.8AU as the nucleus had moved further away from Ulysses during the eight-day travel time. Cometary experts had thought that the molecules and ions that make up a comet's tail would mingle with the solar wind and eventually become indistinguishable.
"We found that the whole thing is preserved as an entity and doesn't spread out very much," says Gloeckler. "The comet is like a point source. It emits neutral atoms and molecules which become ionised by the solar wind as they move away from the nucleus. After several million kilometres, they are all ionised. But instead of then mingling with the surroundings, this ionised sample gets picked up by the solar wind which shoots it out," he explains. By comparing the Ulysses findings with those of the Giotto spacecraft for comets Halley and Grigg-Skellerup, Gloeckler and his team have even been able to determine where the material that they detected originated - in Hyakutake's coma, the diffuse shell of gas surrounding the comet's nucleus. One reason for the tail's survival is probably that it was travelling in the fast solar wind, a steady stream of charged particles flowing out from near the Sun's poles. Comet tails flowing through the more variable slow solar wind, which emanates from near the Sun's equator, are more likely to be disrupted.
"The fast solar wind helped to maintain the magnetic field signature over such a large distance. If it can persist as far as Ulysses, there's no reason to presume that it wouldn't continue to the edge of the heliosphere (the boundary about 100AU from the Sun between the solar wind and the interstellar medium)," says Jones. "This discovery makes us wonder whether Ulysses or other spacecraft have crossed a comet tail before. So we're going back to look again for other signatures. But it's probably a rare event," says Jones. The comet nucleus has to be in exactly the right position with respect to the Sun and the spacecraft for the tail to pass over the spacecraft at the right time - and the chances of that happening very often are probably small.
Ulysses results inspire a big discovery about the Sun's behaviour
The strength of the Sun's magnetic field has doubled during the 20th Century, according to calculations by British scientists. This finding will help to clarify the Sun's contribution to climate change on the Earth. A team at the Rutherford Appleton Laboratory near Oxford has been able to work out the recent history of the Sun's magnetic behaviour, thanks to the unprecedented overview of solar magnetism provided by the ESA-NASA spacecraft Ulysses.
"One surprise led to another," says Mike Lockwood, lead author of the new report. "Ulysses found that the radial component of the magnetic field far out from the Sun is equally strong at all solar latitudes. Nobody expected that, but it means we can use historical data from just one place, the Earth, to deduce a surprising change for the whole Sun. The Ulysses result was absolutely crucial."
Launched in 1990 and still going strong, Ulysses is the first spacecraft ever to pass over the polar regions of the Sun. It revealed that the solar wind of electric particles is generally much faster than that coming from the Sun's equatorial regions, which supply the solar wind felt in the Earth's vicinity. But the strength of the magnetic field carried by the solar wind remains stubbornly constant. Over the Sun's south pole in 1994, and in a quick transit to the north pole in 1995, Ulysses showed that the solar wind smooths out an expected intensification in the polar regions.
Encouraged by this result, Lockwood and his team re-examined a record of magnetic storms on the Earth provoked by the Sun. Called the "aa" index, it comes from simultaneous observations of magnetic events in England and Australia. The Rutherford group found that recent values of the index match very closely the variations in the strength of the solar magnetic field as measured by spacecraft. Since 1964, the magnetic field has intensified by 40 per cent.
As the longest of all records in solar-terrestrial physics, the "aa" index goes back to 1868. Repeated increases and falls correspond roughly with the cycles of sunspots counted on the Sun's visible surface. More remarkable is a rising trend in the index through most of the 20th Century. The Rutherford team deduces from the trend an overall increase in the Sun's magnetic field by a factor of 2.3, since 1901.
Eugene Parker of the University of Chicago, the father of solar-wind theory, comments on the result: "It is a historical fact that our capricious climate responds to variations of the Sun's magnetic activity, with substantial warming and cooling with the rise and fall of activity over the centuries." In Parker's opinion the new discovery about solar magnetism should prompt fresh attention to the role of the Sun in contemporary climate change, alongside any effect due to man-made carbon dioxide.
The Rutherford team itself is already using the magnetic data to deduce increases in the Sun's brightness during the 20th Century. Other work in Europe on solar influences on climate includes studies of stratospheric effects (Berlin and Leicester) and of changes in cloud cover apparently associated with variations in cosmic rays, which obey changes in the solar wind (Danish Space Research Institute).
The paper, "A doubling of the Sun's coronal magnetic field during the past 100 years" by M. Lockwood, R. Stamper and M.N. Wild, is published in the journal Nature, 3 June 1999, vol. 399, pp. 437-9. The comments by E.N. Parker are in the same issue, pp. 416-7.
Ulysses scientists try to catch the wind
More than 50 scientists met at the European Space Research and Technology Centre (ESTEC) in Noordwijk, Netherlands, on 14, 15 and 16 April to discuss the latest scientific results to come from the Ulysses out-of-ecliptic mission.
Meeting for the 41st time in its long history (the first meeting took place over 20 years ago at JPL in May, 1978), the Ulysses Science Working Team (SWT) reviewed the status of the mission, and actively debated the recent scientific findings reported in more than 30 presentations.
Of particular interest was the question "Has Ulysses once again caught up with the fast solar wind?" Presently located some 25 degrees below the Sun's equator, Ulysses has reached the latitude at which, earlier in the mission, it became immersed in fast flowing solar wind from the polar regions. All indications are, however, that this time, the boundary between the fast polar wind and slow wind from the equatorial belt is out-running the spacecraft as it heads poleward. Scientists expect that the well ordered pattern of fast and slow wind that Ulysses encountered near solar minimum will give way to a more chaotic situation as solar activity increases. The continued absence of prolonged periods of fast wind at Ulysses seems to bear this out.
Another Ulysses experiment has detected samples of solar wind material that have been absorbed by dust particles in the inner heliosphere, only to be re-emmited at a later time to become a separate population of energetic particles.
Ulysses Aphelion Workshop Held in Oxnard, California, 27-30 October 1998
The 3.5-day Ulysses Aphelion Workshop, held in Oxnard, CA on 27-30 October and attended by about 50 scientists, was organised around four themes: Data comparisons at 1 and 5 AU; Science goals for solar maximum; Shocks, waves, particles and turbulence; Interstellar/astrophysical aspects of Ulysses. Separate Working Groups, lead by one or more members of the Ulysses science team, were established in advance of the workshop for each of the themes, and participants were encouraged to provide input to the Working Groups prior to the meeting. This was facilitated by the creation of dedicated Workshop Web pages on the Ulysses/ESA Web site, which will continue to be used to provide progress reports on activities that were initiated as a result of the workshop. Although no formal proceedings of the workshop will be produced, it is anticipated that the highly successful meeting will result in a large number of papers appearing in the literature.
Ulysses and the "Mystery Force"
Recent reports have created a stir among scientists studying the effects of gravity. A team lead by John Anderson, a planetary researcher at the Jet Propulsion Laboratory in Pasadena, has been conducting an experiment in celestial mechanics using the radio signals from spaceprobes far from the Earth, including Pioneers 10 and 11, and Ulysses. In all cases, Anderson and his colleagues found that an unexplained Sun-directed force appears to be acting on the spacecraft. This apparent deviation is tiny - equivalent to an acceleration of 120 billionths of a centimetre per second squared in the case of Ulysses - and has no impact on spacecraft operations. If a force of this size were applied to a car moving at 60 km/hr, it would take 650 years to come to a standstill! Nevertheless, for scientists studying the detailed aspects of gravitational theories, it presents a puzzle.
As John Anderson himself admits, despite a very thorough analysis, the most likely explanation is a systematic effect in the radio data that has been somehow overlooked. If a real "mystery force" is involved, it should also affect the orbits of the planets, and this can be almost certainly ruled out. Another exotic candidate, dark matter, is equally unable to account for the observations.
Until someone can come up with definitive proof that the data are in error, the possibility remains that Anderson and his team have uncovered a new physical phenomenon. "There's a small probability that it's very important," says Anderson. [Extracted from New Scientist, 12 September, 1998].
Latest on the "Mystery Force"! (16 October, 1998)
A possible explanation for the mysterious "slowing down" of Ulysses and other interplanetary spacecraft has been offered by physicist Jonathan Katz and astronomer Edward Murphy: Ulysses, Pioneer 10 and 11, and Galileo all use radioisotope thermoelectric generators (RTGs) to generate their electrical power. Excess heat from the RTGs is radiated into space, mainly in a direction away from the Sun. The amount of radiation emitted could account for the "push". [Extracted from New Scientist, 17 October, 1998].
Ulysses Eight Years in Space!!
Eight years ago today, Ulysses was launched by the space shuttle Discovery from launch pad 39B at Cape Canaveral on STS-41. Ulysses was subsequently sent on its way to Jupiter by the combined power of the upper stage IUS and PAM-S rockets.
On this date the Gamma-Ray Burst (GRB) experiment on board Ulysses detected an exceptional flash of gamma rays coming from the star known as Soft Gamma Repeater SGR 1900+14. Scientists believe that SGRs are rapidly spinning neutron stars with incredibly strong magnetic fields. A neutron star is the super-dense object left over when a star explodes as a supernova. At least six other spacecraft, including the Italian Beppo-SAX and NASA's Rossi X-ray Timing Explorer, confirmed the signal.
|The Ulysses measurements, plotted as a light curve, show an intense flash of radiation followed by a gradual decline in the gamma-rays. This Ulysses measurement will become the classical light curve for these highly magnetized stars or magnetars. Ulysses light curve courtesy of Kevin Hurley, GRB Principal Investigator, University of California, Berkeley.
|The relative positions of Ulysses and the six other spacecraft which recorded the gamma ray burst from SGR 1900+14. SGR 1900+14 itself lies 20,000 light years beyond our solar system. To put this in perspective, if our solar system were shrunk to the scale of this diagram, SGR 1990+14 would then lie 24,000 km (15,000 miles) away.
[ GRB Instrument ]
The distance between Earth and Ulysses reaches its maximum value this month. On 28 August, the spacecraft and its home base are separated by 951 million kilometres (6.36 times the Sun-Earth distance). At this distance, signals from the spacecraft take 53 minutes to reach the Earth.
This month, Ulysses completed its first full out-of-ecliptic orbit of the Sun. On 17 April, 1998, more than 6 years after the close flyby of Jupiter (8 Feb 1992) that deflected the spaceprobe out of the plane of the planets and over the poles of the Sun, Ulysses returned to the point in space where its climb to high latitudes began. This time, however, Jupiter was far away on the opposite side of the Solar System, so the path followed by the spacecraft will not be changed.
On 14 December, 1997, Ulysses crossed the heliographic equator for the third time, this time heading southwards. Previous crossings were in December 1990 (N to S) and March 1995 (S to N).
The Report of the 1997 NASA Senior Review of Sun-Earth Connection Operating Missions has been released. The Review Panel was very positive in its assessment of Ulysses, and concluded "ULYSSES is our one scientific opportunity to explore and understand the conditions in the heliosphere at high latitudes and its relation to low latitude activity, at all levels of solar activity. It is imperative that this exploration ... be pursued vigorously."
Nine investigations have been selected to participate in the Ulysses Guest Programme.
Nine Letters of Intent to propose for the Guest Investigator Programme have been received in response to the ESA AO.
The Ulysses Guest Investigator Programme Announcement of Opportunity was released by ESA on 20 June, 1996.